High impedance, low current faults, such as a downed distribution line conductor which is contacting a poor conductive earth composite, have proven to be difficult to isolate with present technology. Conventional overcurrent protection devices, both at the source and at strategic circuit locations, use the combined measurement of fault current magnitude and time duration to clear faults associated with downed grounded conductors.
Potential serious problems caused by a high impedance fault in an electrical distribution system include:
(1) a live bare wire or one with damaged insulation which is in contact with earth and is remotely located from the source; PA1 (2) a live bare or insulated conductor downed and broken and in contact with a poor conducting medium, i.e., sand, rock, concrete, snow, blacktop or a tree; PA1 (3) a live conductor broken and hanging above earth, with the load side of the conductor backfiring through a three phase power transformer; or PA1 (4) a live conductor down, but intact, and grounded through a poor conducting medium such as listed in item 2 above.
For reliability purposes, it is common practice to install downstream circuit reclosers, expulsion fuses or sectionalizers at all taps to the main stem distribution circuit. The intent of the application of these protection devices is to locally isolate downed faulted conductors in the smallest sections possible, yet maintain normal service to the balance of the customers on that same circuit. These downstream over-current protection devices are designed to be time coordinated with each other and the main circuit breaker in order to automatically isolate the downed primary conductor. Overcurrent protection devices are unable to distinguish low fault currents (high impedance faults) from normal load currents because trip settings for these devices are typically set at 125 to 250 per cent of maximum estimated peak load current. These levels are selected to minimize inadvertent tripping.
A hazardous condition for the public is created when energized high voltage conductors fall to the ground or come in contact with a high impedance return path and the overcurrent protection system fails to de-energize the conductor. Physical contact with an energized distribution primary conductor by any conducting body may cause serious injury or death due to electric shock. Numerous fatalities and serious injuries occur annually in the United States due to inadvertent contact with live downed power distribution conductors. Experience has shown that these conditions occur more frequently at distribution level voltages below 15KV, which is the predominant primary distribution voltage range in the United States.
Over the years, several high impedance fault detection systems have been developed. These techniques vary in their approach, but most monitor various arcing fault-generated harmonics rather than the fundamental frequency. There are a wide range of factors influencing the type and magnitude of the harmonics which may occur in this type of fault condition. In fact, certain wire down conditions may result in no arcing due to the insulating characteristics of ground material that the conductor has contacted, thus no harmonic is generated.
The technique of sensing high frequency components from arcing faults has been hindered by the fact that distribution feeder capacitor banks block the high frequency signal from the monitoring location. In addition, the source of the harmonic is not easily located on a grid system, as these signals are transmitted via the path of least impedance. In addition, an arcing high impedance fault may last intermittently for several seconds or even minutes if the involved conductor remains energized. The high impedance, low fault current condition also generally doesn't affect the stability of the power system. However, this type of fault creates a potentially large liability for the electric utility due to possible fire or electrocution.
A common prior art approach for detecting high impedance fault currents monitors a number of harmonic frequency components of the combined load and fault current on the electrical distribution circuit. The harmonic data is gathered and compared with a pattern which is characteristic of a normal system. This waveform data is analyzed in backup relay circuitry to operate the main feeder circuit breaker if an overcurrent sensing device has cleared the downed conductor.
An example of a prior art high impedance fault sensing arrangement is shown in the simplified schematic diagram of FIG. 1. An overhead distribution primary circuit 10 experiences a high impedance fault 12 on a branch tap 16 not detectable by a circuit reclosure 14 or a main overcurrent relay-circuit breaker combination 18. A high impedance detection arrangement 20 receives generated signals through a transducer 22. The signal is conditioned and compared by a microprocessor 24 with a stored signal pattern which is characteristic of normal system operation. A micro-computer 26 makes a trip-output decision based upon several operating parameters which are weighted.
A high impedance fault isolation system is needed for electrical utilities to minimize the time period that a downed wire remains alive, after an overcurrent protection device has failed to de-energize the downed live wire. It can also provide a higher quality of service to other customers on the same distribution circuit by isolating a high impedance fault and permitting normal service to continue on the remainder of the circuit.
The present invention overcomes the aforementioned limitations of the prior art by sensing loss of voltage on the load side of a downed conductor rather than an overcurrent situation in detecting a high impedance fault condition. The detection and isolation of the downed live conductor is automatically analyzed and controlled by a host computer through remote tripping of an isolation device. This process occurs automatically and serves as a backup to an overcurrent protection system for de-energizing high impedance electrical distribution system primary faults.